It's a comparatively new industry term, and represents a variable which may be written to (precisely) once.
MarkMLl
Yes, I know. From functional languages, but different.
Like querying a read-only database: you can only create new values by mutating old ones. Like, a view with one or more calculated fields that reference other fields. Or, like Lisp.
Generally, if you have a read-only property, you set it once, in the constructor. The constructor has a formula to calculate it from other values. Once.
Sounds like the same thing to me.